1. Field of the Invention
The invention relates to the taking of core samples from geological reservoir formations. More particularly, it relates to obtaining a balanced, pressurized core sample from an underpressurized geological reservoir formation using a stable foam as a drilling fluid and pressurizing fluid.
2. Description of the Prior Art
Pressurized core samples are useful for obtaining residual saturations and in-situ gas-oil ratios in deep geological reservoirs. U.S. Pat. No. 3,548,958 discloses a pressure core barrel for taking pressurized core samples. U.S. Pat. No. 3,463,231 discloses a method for drilling wells using foam as a drilling fluid. A paper by Alan L. McFall entitled "Recent Developments in Pressure Coring", paper SAND80-0253C presented at the Energy Sources Technology Conference and Exhibition, New Orleans, Feb. 3-7, 1980; discloses the taking of pressurized core samples using a brine-based polymer as a coring fluid. A number of papers discuss well drilling using foam as a drilling fluid. For example: Lorenz, Howard, "Air, Mist and Foam Drilling Has World-Wide Application", World Oil (June 1980), page 187-193; Krug, Jack A. and Mitchell, B. J., "Charts Help Find Volume, Pressure Needed for Foam Drilling", Oil and Gas Journal, (Feb. 7, 1972), page 61-64; and Millhone, R. S., Haskin, C. A., and Beyer, A. H., "Factors Affecting Foam Circulation in Oil Wells", paper SPE 4001 presented at SPE 47th Annual Fall Meeting, San Antonio, Oct. 8-11, 1972.
For the purposes of this invention and as used in the specification and claims, a stable foam is a completely mixed gas and liquid dispersion where the liquid is the continuous phase and the gas is the discontinuous phase. Under normal conditions, a stable foam is a compressible fluid which behaves as a Bingham plastic fluid, as discussed in the above reference authored by Krug and Mitchell.
Unique problems, however, must be overcome before a pressurized core sample can be taken using foam as a core drilling fluid. A major problem stems from the fact that pressure balance is an absolute requirement for successful pressure coring. Too large a pressure overbalance during coring operations causes a flushing of the core with mud filtrate, thereby altering the core saturations and properties. Underbalance of pressure during coring operations causes an exuding of core fluids, also altering the in-situ saturations. Thus, precise foam behavior at the well bottom (i.e. bottom hole) during pressure coring operations must be known. However, there has heretofore been no method for precisely predicting the foam behavior as a function of the foam composition and a control pressure applied to the foam. Without an accurate method of accomplishing this critical step, reliable and practical pressure coring using foam cannot be performed.